Historically, zinc has been produced in retorts, in electrolytic cells and in the Imperial Smelting Furnace (ISF). Today almost all zinc is produced by the electrolytic process or the ISF process. The ISF is used to treat dirty and low grade concentrates as well as some secondary materials. To some extent its economic viability is threatened because of its large usage of high cost coke, its low thermal efficiency and the need for separate sintering and refining stages. To an extent the economics of the electrolytic process are dependent on low electricity costs. The process also produces a residue that presents disposal problems.
This invention constitutes an advantageous alternative to the ISF and has the advantages of better utilisation of exothermic heats of reaction, use of low cost coal as reductant instead of metallurgical coke, and production of a high calorific value by-product gas.
The KIVCET process (Australian Pat. Nos. 421,261; 456,550) has been used commercially to treat dirty copper concentrates, and in a modified form has been piloted to treat lead concentrates including some containing amounts of zinc. It has not been applied commercially to materials in which zinc is the predominant metal.
The copper variant of the KIVCET process involves cyclone smelting of dried concentrates with oxygen to produce a molten bath containing any lead and zinc present in oxidised form as a slag and the copper as a matte. The sulphur dioxide off-gas is of high tenor and can be used for acid production. Slag and matte pass under a partition wall into an electrothermic furnace zone where reducing agents are added, to the bath. Heat is provided by electricity. Lead metal may be recovered from the slag, zinc may be fumed off and copper matte remains unchanged. The latter is transferred to a converter to produce blister copper.
The KIVCET process for lead is similar, but is not considered suitable for adaptation to predominantly zinc containing materials unless low cost electricity is available on account of the high energy requirements involved.
The Outokumpu company in Finland has investigated the smelting of lead and is also studying the fuming of zinc from slag produced in lead smelting. According to Outokumpu (Australian Pat. No. 501,911), concentrates are flash smelted to produce molten lead metal and a liquid slag which are passed to an electrothermic furnace where zinc is fumed off by injecting fine coal entrained in nitrogen. When applying this process to materials containing high percentages of zinc, a large amount of heat is produced as a result of the oxidation of zinc sulphide. In flash smelting, this heat is immediately used up in the volatilisation or large amounts of lead which gives rise to a large unwanted circulating furnace load.
In the QSL process (U.S. Pat. No. 4,266,971), developed by Lurgi in West Germany, a molten bath of slag is fed with pelletised lead concentrates. The pellets enter the bath into which oxygen is injected. A high lead oxide slag is produced which is subsequently reduced to metallic lead in a second part of the furnace by injecting coal. As far as is known, this process has not been developed for the fuming of zinc.
The ISASMELT process (Australian patent application No. 90762/82), is another bath smelting process for the smelting of lead concentrates. It consists of adding lead sulphide to a molten slag, injecting air into the slag bath to agitate the bath and to oxidise the lead sulphide to lead oxide and subsequently reduce the oxide to lead metal. Any zinc in the lead concentrates and remaining in slag, may be recovered by the addition of a slag fuming stage to the process.
A process which is aimed at predominantly zinc containing raw materials has been proposed by Davey and Yazawa. The process concept involves the following reaction at elevated temperatures EQU ZnS+O.sub.2 =Zn+SO.sub.2
As this reaction is endothermic, addition of carbonaceous fuel is required resulting in a mix of carbon oxides and sulphur dioxide in the off-gases. This makes it difficult to recover the heat value in any unburnt carbon monoxide and may lead to thermal inefficiency. There is also a potential problem of recovering zinc vapour from the complex gas stream.